The invention relates to a cold trap for eliminating impurities from a polluted liquid metal. A liquid metal, e.g. sodium, potassium or lithium, or one of the alloys thereof, circulating as a heat exchange medium, is subject to the formation of impurities in the form of oxides, hydrides, hydroxides of metallic particles and carbon.
Cold traps are incorporated into the circuit of said liquid metal to eliminate the impurities therefrom. In the case of a nuclear reactor, they exist on the primary circuit and on the secondary circuit. In these traps, the metal is cooled to below the soluble impurity saturation temperature. For this purpose, they have an external cooling device for cooling the liquid metal from the intake temperature to a temperature below the impurity saturation of the liquid metal, so as to mainly crystallize the soluble impurities. Moreover, the cold traps generally have a recuperation-type heat exchanger or recuperator, in which the liquid metal to be purified and which enters the trap gives of part off its heat to the purified liquid metal leaving the trap. Finally, they have lining members forming a porous medium to hold back the crystallized solid matter and filter the insoluble particles.
A cold trap of this type is already known (FR-A-No. 1 549 434). It comprises a vertically positioned cylindrical body, which is closed in its upper part and in its lower part by a convex base. Within said body is provided a cylindrical ferrule arranged coaxially to the body, so as to define an annular space with the inner wall of said body. A lining member is placed within the ferrule. The polluted or contaminated liquid metal is introduced into the upper part of the body and circulates in the annular space between body and ferrule.
A casing outside the body and in which is ensured a flow of the cooling fluid, makes it possible to cool the lower part of the body and consequently reduce the temperature of the liquid metal to below its impurity saturation temperature. The impurities are normally held back on the lining member.
The liquid metal then rises in the interior of the ferrule and passes through a heat exchange circuit with the polluted hot liquid metal. It is then discharged from the body.
The present invention relates to a cold trap having improved performance characteristics compared with the prior art cold traps.
It is firstly advantageous to increase the retention capacity of a cold trap. Thus, during its operation, the cold trap is progressively filled with impurities. When the trap is filled, it must be emptied or changed, so that it can again fulfill its function in a satisfactory manner. Obviously on increasing the retention capacity of a cold trap, the number of traps necessary for purifying a given sodium quantity would be smaller. Thus, the traps would have to be emptied less frequently.
In particular in the case of an integrated fast neutron nuclear reactor, i.e. of the type in which the exchangers and pumps of the primary circuit are located within the vessel and are immersed in a liquid metal volume, the traps of the primary circuit can be advantageously located in the main vessel. Obviously, in order to reduce the size and dimensions of the vessel, the components located in the latter must be as small as possible. It would therefore appear desirable to design a cold trap having reduced dimensions, so that it can be advantageously located in the vessel of an integrated nuclear reactor, whilst also havina a large retention capacity, which would appear to be contradictory in the light of the prior art.
The present invention relates to a cold trap making it possible to solve this problem.
Moreover, particularly in the case of a cold trap of the primary circuit of an integrated fast neutron nuclear reactor, the duration of a purification campaign for a given mass of liquid metal also constitutes a very important factor.
Thus, it is known that the mass of liquid metal is exposed to two different types of pollution. A first pollution type is released at the start of the operating cycle by fuel assembly surfaces. This pollution occurs once during the life of the assemblies. It constitutes a mixed hydrogen--oxygen pollution. There is also a continuous pollution by hydrogen and tritium linked with the operation of the reactor.
It is known that hydrogen and tritium are liable to diffuse towards the secondary circuits through the walls of intermediate exchangers. The continuous hydrogen and tritium pollution could thus be eliminated by cold traps of secondary circuits in the form of hydrides.
However, the oxygen pollution must be eliminated by a primary circuit cold trap in the form of oxides and it is advantageous to perform this purification as quickly as possible, at the start of the cycle, before excessive quantities of hydrides have had a chance to be fixed in the trap.
Moreover, if there is a sodium--water reaction in a secondary circuit, it is necessary to rapidly purify the liquid metal to reduce risks of clogging and corrosion at certain points of the circuit.
The present invention also relates to a cold trap making it possible to reduce the duration of a purification campaign as a result of increases efficiency.